In processing, particularly precision processing such as cutting, grinding or polishing of metal plates, molds, aluminum sashes, plastic plates, semiconductor wafers, circuit boards, ceramics, optical components such as glass and quartz, and electric and electronic components such as sensors, a surface protective film to temporarily protect components has been widely used so as to prevent damage by cutting water, scars or contamination by e.g. cutting wastes, or breakage, by covering an already processed surface of a member to be processed and a functional portion such as a circuit or a sensor portion. As such a surface protective film, a pressure-sensitive adhesive sheet has been mainly used.
Further, in plating process employed for a circuit board such as a printed-wiring board, a pressure-sensitive adhesive sheet cut into an optional shape, which is a protective material to prevent contamination by a plating liquid, has been used as a masking material to a portion on which plating is not required on a circuit.
Further, in addition to electric and electronic components, painting and various printings are conducted by various printing methods on various industrial products, such as tablets such as nameplates and logos, scales on measuring instruments and decorations. In a case where there is a portion on which painting or printing is not required, a pressure-sensitive adhesive sheet is bonded to that portion and then painting or printing is carried out.
However, in a case where a pressure-sensitive adhesive sheet is used as a protective film when a member to be processed is processed, along with downsizing and miniaturization of components in recent years, the bonding operation is complicated and in addition, even a slight displacement of the bonding position may impair the yield or the design properties of the products. When a component has recesses and protrusions, the sheet cannot follow the recesses and protrusions and is removed during processing, thus contaminating the member to be processed in some cases. In addition, when a pressure-sensitive adhesive sheet which has been cut to be fitted to the shape of the member to be processed is to be used, an expensive mold is required to cut the sheet, such being disadvantageous in view of cost.
In order to solve these problems, separately from a pressure-sensitive adhesive sheet, a masking method has been studied, comprising coating a surface of a member to be processed with an ultraviolet-curable composition capable of being removed by a specific organic solvent or aqueous alkali solution by screen printing, followed by curing with ultraviolet rays for covering (Patent Documents 1 to 3).
However, since the method employs an aqueous alkali solution or an organic solvent, the cleaning process is complicated, and the method is problematic in view of working environment. Further, in a case where the surface of a member to be processed has fine irregularities, the organic solvent cannot sufficiently infiltrate, whereby the protective film cannot completely be removed, thus causing problems in the outer appearance of the member to be processed.
Further, making a semiconductor wafer or an optical component thin (hereinafter sometime referred to as “grinding”) has been carried out by backgrinding wherein a circuit surface of the wafer or the optical component is protected with a surface protective sheet, and the back side opposite to the circuit surface is ground.
At present, the thickness of wafers is usually 150 μm at the industrial level. However, further thinner wafers have been desired. In a case where a wafer is made thinner, a phenomenon such that the ground side (back side) has irregularities by the influence of the recesses and protrusions on the circuit surface, i.e. back transfer of a circuit pattern is remarkable.
The reason why the back transfer occurs is explained as follows. Namely, a pressure-sensitive adhesive surface protective sheet presently used has limited capability to follow recesses and protrusions of a circuit on a semiconductor wafer. Accordingly, a gap (air pocket) is present between the adhesive layer and the circuit surface, and the wafer is not directly supported by the adhesive (protective layer) at that region. When the wafer is made thin by grinding, at the scribe line (street) not supported, the wafer moves in the vertical direction between circuit dies while the air pocket is compressed and resultingly, the wafer is not ground in this region and becomes thicker than the other portion. On the other hand, in a case where there are hard protrusions such as bumps, the wafer is further ground and becomes thinner at such a region than the other portion resultingly.
The above phenomenon is not problematic when the thickness of the finished wafer is 150 μm or more, but when the wafer is thinner than 100 μm (particularly when the wafer is finished to have a thickness of 50 μm or below) or when protrusions on the wafer circuit surface such as bumps are remarkably high (for example, 100 μm or more), not only the deflective strength of the wafer will be remarkably lowered, but in a worse situation, the wafer may be even broken during grinding.
Further, when the wafer is ground to make it thin at a level of 50 μm, the cutout of the edge of the wafer or infiltration of the grinding water into a space between the wafer and the surface protective layer are problematic. This is also due to insufficient adhesion of the surface protective sheet to the edge of the wafer. Further, along with the tendency to make the wafer thin, on a semiconductor wafer which has, on the circuit surface, protrusions of 100 μm or higher represented by bumps, typical grinding itself carried out by bonding a semiconductor surface protective sheet is difficult.
A conventional surface protective sheet is usually a sheet comprising a polymer film material and an adhesive layer as a surface protective layer on the polymer film material. The adhesive is designed to have a low elastic modulus so as to follow the recesses and protrusions on the circuit surface. However, if this tendency is too significant, a great stress will be applied to the wafer when the sheet is removed by peeling from the wafer, thus leading to breakage. Accordingly, a protective sheet which will be easily removed by energy rays has been developed, wherein the adhesive is cured by irradiating with energy rays such as ultraviolet rays before peeling of the sheet to reduce the adhesive force between the wafer and the protective sheet. However, the adhesive layer is too soft if it is uncured at the time of grinding, whereby the wafer may be broken during grinding.
Patent Document 4 discloses a wafer grinding method which comprises bonding the above protective sheet which will be easily removed by energy rays to a wafer on which a circuit is formed, curing the adhesive layer with energy rays, and then carrying out backgrinding of the wafer. However, the adhesive, which is not a fluid, has insufficient capability to follow the recesses and protrusions on the wafer circuit surface.
Further, Patent Document 5 discloses a hot-melt type semiconductor surface protective sheet. A hot-melt type sheet which is melted by heating at from 60 to 100° C. and shows fluidity, follows the recesses and protrusions on the circuit surface and exhibits excellent grinding properties. However, this sheet is repeatedly melted every time the temperature exceeds the melting point.
A semiconductor wafer after bonded to a protective sheet, usually undergoes heating in a step of bonding a film to be used to fix a chip i.e. a die attachment film (hereinafter sometimes referred to as “DAF”) or in a step of forming a metal film by sputtering in some cases. Resultingly, such a defect may occur that the protective sheet is re-melted in some cases.
Further, separately from the surface protective film, a method has been studied to protect a member to be processed from e.g. cutting wastes during processing, by coating the surface of a member with an ultraviolet-curable adhesive which is soluble in a specific organic solvent, followed by curing with ultraviolet rays for covering. However, since an organic solvent is used, the cleaning step is complicated, and there is a problem in view of working environment. Further, in the case of fine recesses and protrusions, the organic solvent cannot sufficiently infiltrate to completely remove the protective film, thus causing problems in the outer appearance of the member to be processed.    Patent Document 1: JP-A-59-051962    Patent Document 2: JP-A-01-234477    Patent Document 3: JP-A-03-139573    Patent Document 4: JP-A-11-026406    Patent Document 5: JP-A-2000-038556